Redux broken krb5.h handling (#1156)

Redux broken krb5.h handling (#1156)

De-duplicate the #include sequence into a libcompat header to
ensure all uses of the hack(s) including autoconf test macros
are using the same logic. This fixes some macros which were
using inconsistent logic for Solaris.

Simplify the if-elif use of __cplusplus detection. All the
extern C hacks apply only when C++ is used.

Wrapping the entire file peer_proxy_negotiate_auth.cc with
'extern C {...}' is unnecessary (it is a .cc) and breaks the
Solaris workaround.